Insulating coating comprising an aqueous mixture of the reaction product of chromium nitrate and sodium chromate,phosphoric acid and colloidal silica and method of making the same

ABSTRACT

ELECTRICALLY INSULATING COATING FOR STRAND ANNEALED MAGNETIC SILICON STEEL IS COMPOSED OF AN AQUEOUS MIXTURE OF (1) REACTION PRODUCT OF CHROMIUM NITRATE AND SODIUM CHROMATE, (2) PHOSPHORIC ACID AND (3) COLLOIDAL SILICA.

United States Patent Ofiice Patented Feb. 9, 1971 1: 3,562,011 INSULATING COATING COMPRISING AN AQUE- OUS MIXTURE OF THE REACTION PRODUCT OF CHROMIUM NITRATE AND SODIUM CHRO- MATE, PHOSPHORIC ACID AND .COLLOIDAL SILICA AND METHOD OF MAKING THE SAME Robert G. Hirst and Daniel W. McMorris, Pittsfield,

Mass., assignors to General Electric Company, a corporation of New York No Drawing. Filed Apr. 26, 1968, Ser. No. 724,636 Int. Cl. C04b 35/14 US. Cl. 117-234 15 *Claims ABSTRACT OF THE DISCLOSURE Electrically insulating coating for strand annealed magnetic silicon steel is composed of an aqueous mixture of (1) reaction product of chromium nitrate and sodium chromate, (2) phosphoric acid and (3) colloidal silica.

The present invention relates to coated magnetic sheet material and a method of making the same. More particularly, the invention concerns refractory coating material which is applied to such sheet material, and especially to strand annealed silicon steel strip, and serves as a permanent insulating coating in the use of the metallic sheet material in laminated electrical structures.

The forms of magnetic sheet material with which the invention is concerned include strip material such as used in wound transformer cores, and cut or punched laminations forming stacked transformer cores or other electrical apparatus. Examples of compositions of magnetic sheet materials which may be effectively coated in accordance with the invention are silicon steel (typically containing about l5% silicon), nickel-iron alloys, common iron, and other ferromagnetic materials.

The invention is particularly applicable to silicon steel strip which has been subjected to a grain growth and purification anneal treatment wherein the steel is heated in the form of a single strand, as distinguished from coiled or stacked form, in a suitable furnace through which the strand may be continuously passed during the heat treat- .ment.

In the past, it has been the practice to subject the steel sheet material in coiled or stacked form to the annealing treatment, and in order to prevent sticking of the sheet surfaces to each other during such elevated heat treat ment, an insulating coating material such as magnesium hydroxide has conventionally been applied to the sheet material prior to such annealing stage. Such coating material has served both to avoid the problem of sticking during the anneal and to provide electrical insulation to reduce eddy current loss in the laminated core formed by the sheet material in its use in a transformer, motor or the like. However, the application of insulating coatings in general to the steel sheet prior to the annealing treatment has the disadvantage in a strand anneal procedure that the presence of the insulating coating tends to delay the purification of the steel during the strand anneal, e.g., by hindering removal of sulfur from the steel, and consequently the strand anneal process is not as rapid as it might be in the absence of such coatings. Moreover, the prior types of inorganic insulating coating such as magnesium hydroxide require relatively high curing temperatures, e.g., over 1000 C., to produce final coatings of the requisite hardness, electrical insulating and other properties. While such elevated temperatures are employed during the usual grain growth and purification anneal, it has in many cases been found desirable to apply one or more additional insulating layers after the usual anneal and for this purpose the sheet material has to be heated again to elevated temperature to cure the added insulating layer.

It is, therefore, an object of the invention to provide an improved insulating material for metallic sheet material which avoids the above-mentioned disadvantages.

A further object of the invention is to provide an improved insulating coating composition for the above purposes which requires relatively low curing temperatures.

Still another object of the invention is to provide an insulating coating composition which is particularly adapted for use on electrical steel sheet material treated by a strand anneal process.

It is another object of the invention to provide an improved method of making insulated steel sheet material of the above type.

Other objects and advantages will become apparent from the following description and the appended claims.

With the above objects in view, the present invention in one of its aspects relates to an insulating coating composition comprising a mixture of water soluble salt of trivalent chromium, a water soluble chromate salt, phosphoric acid, a finely divided refractory material selected from the group consisting of colloidal silica and colloidal alumina, and water. In a preferred form of the invention, the chromium and chromate salts are initially mixed together to form a reaction product which is then mixed with the remaining ingredients of the mixture.

In a typical application of the present invention, the foregoing insulating mixture is deposited on the surfaces of silicon steel strip which has been subjected to a strand anneal process for producing the desired magnetic texture and removing undesired impurities therein. Such a strand anneal process is disclosed in detail, for example, in Fitz et al. Pat. 2,986,485, and the disclosure of that patent is incorporated herein by reference. As there disclosed, the strand anneal process, which may be carried out on a continuous basis, comprises the steps of heating silicon steel material for about 5 to 60 minutes (preferably about 15 minutes) at a temperature of about 950 to 1050 C., and thereafter heating the material at a temperature of about 1175 to 1250 C. for about 1 to 30 minutes (preferably about 15 minutes). The annealing treatment is preferably carried out in a reducing atmosphere in the furnace, such as in hydrogen or other reducing gases. In accordance with the invention, the thus annealed steel strip, which previous to the anneal had not been provided with an insulating coating, is then coated with the abovedescri'bed mixture, and the latter is then dried by heating or otherwise cured to form a hard, tightly adherent, highly insulating coating on the steel surfaces.

A particular coating mixture found eminently satisfactory for the purposes of the invention has the following composition, shown in a preferred and a range of proportions in percent by Weight:

Range, Preferred,

percent percent Reaction product (dried powder) of:

(1) Chromium nitrateCr (N Oa)a-9HzO In a typical procedure, there is initially prepared a mixture of the chromium nitrate and the sodium chromate in water", the solution being stirred until the reaction product,- a brown precipitate, forms (about /2 hour). The resulting gelatinous precipitate is filtered, washed with water and dried with acetone. The reaction product thus produced, whose exact composition is presently unknown, is soluble in phosphoric acid, and care must be taken to avoid dehydration, e.g., by excessive heating, since in the dehydrated state the compound is insoluble in phosphoric acid and will not dissolve in the coating solution. It may be added while still wet to the other ingredients, but if dried, it should not be heated above about 100 C.

The hydrated compound thus obtained as the reaction product of chromium nitrate and sodium chromate is then added first to the phosphoric acid, and as the water is added the mixture is stirred until the precipitate dissolves. The colloidal silica is then added to the solution to produce the coating bath. The mixture thus produced is a green liquid with a specific gravity of 1.23 and a pH of about 2. The coating solution is applied to the strand annealed steel material in any suitable manner such as by dipping, spraying, brushing or other methods. Typically, the steel strip, which has been allowed to cool to room temperature, is dipped into the coating bath, allowed to drain 30 seconds, and then baked in an oven at 250 C. for about 1 minute.

The coating thus produced was smooth, hard and tightly adherent and did not craze appreciably on bending of the coated sheet material. It was characterized by excellent electrical insulation properties, had minimal effect on watt-loss and permeability properties, while improving magnetostriction properties. It was found by test, for example, that such a coating deposited in an amount of .05 oz./sq. ft. on the steel sheet provides a Franklin insulation value of about .00 ampere, it being known by those skilled in the art that any Franklin value below .20 ampere is satisfactory for transformer use, and that the lower this value the better the insulating quality.

While chromium nitrate has proved satisfactory for use in the above mixture, other water soluble salts of trivalent chromium may be used, such as chromium acetate, chromium sulfate, and chromium chloride or other chromium halogen salts. Similarly, water soluble chromate salts other than sodium chromate may be used, such as potassium chromate, magnesium chromate, and calcium chromate.

The colloidal silica which may be used in the described mixture is in the form of an aqueous solution containing SiO in an extremely finely divided form made by an ion exchange technique. Colloidal silica is available commercially under the trademark of Ludox, a product of E, I. du Pont de Nemours & Company, which is an aqueous colloidal sol containing approximately 30% Si with less than 0.5% Na O as a stabilizer. A more detailed description of this colloidal silica is found in Robinson Pat. 2,809,137, which description is incorporated herein by reference.

As an alternative to the colloidal silica there may be used colloidal alumina, which is a dispersion in water of alumina particles which are sufficiently finely divided to provide a colloidal sol in water. A colloidal form of alumina available commercially which may be used is sold under the trademark Baymal, a product of E. I. du Pont de Nemours & Company. This is a white, freeflowing powder consisting of minute fibrils of boehmite (AlOOH) alumina and which disperses readily in water to yield a colloidal sol. Further detailed descriptions of this material and colloidal sols thereof and methods of making them are set forth in the patent to Bugosh 2,915,475. In general, for the purposes of this invention a concentration of -50 grams per liter of water of the finely divided colloidal alumina has been found suitable, with a range of 30-40 grams/liter affording optimum results.

While it is preferred to carry out the process by initially forming a reaction product of the chromium and chromate salts as above described, the process may alternatively be carried out simply by mixing all of the ingredients directly without first forming the brown precipitate as above described. A suitable composition for, use in such a process is as follows, in percent by weight:

Range, Preferred,

Although the temperature used for drying thecoating as described above is preferred, the drying stage temperature may vary in. a range of about 200 C. for 3 minutes to about 300 C. for about /2 minute.

The composition of the finally produced coating, which is transparent and pale green in color, is not presently known and cannot be definitely ascertained, and it is accordingly referred to herein simply as the cured product of the mixture of ingredients as above described.

It should be understood that while the described insulating coating is particularly applicable for use on strand annealed silicon steel material, it is not limited to such use. It will be found of advcantage, for example, for coating such steel or other metallic material where rela tively low curing temperatures are or must be used.

There is thus provided in accordance with the invention an improved insulating coating composition which has excellent electrical insulating properties, may be deposited in thin and tightly adherent layers to the steel surface, which does not contribute to electrical loses or other unfavorable electrical properties in the coated steel material, and which is applied easily in a continuous operation. The very tenacious adherance of the insulating coating is especially noteworthy in view of the very smooth surfaces which characterizes strand annealed steel, as distinguished from steel annealed in coiled or stacked form as in conventional processes, and which ordinarily make it difiicult for conventional insulating coatings to firmly adhere thereto.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An insulating coating composition comprising an aqueous mixture of (l) the phosphoric acid soluble, water-Washed precipitate formed by stirring the aqueous mixture of a water soluble salt of trivalent chromium and awater soluble chromate salt, (2) phosphoric acid, and (3) a finely divided refractory material selected from the group consisting of colliodal silica and colloidal alumina.

2. A composition as defined in claim. 1, comprising in percent by weight of about .5 to 10% of said precipitate, about 15 to 30% posphoric acid, about 10 to 40% of said finely divided refractory material, and about to 70% water.

3. A composition as defined in claim 2, comprising about 1% of said precipitate, about 27% phosphoric acid, about 30% colloidal silica, and about 42% water.

4. An insulating coating composition comprising a mixture of (1) a water soluble salt of trivalent chromium, (2) a water soluble chromate salt, (3) phosphoric acid, (4) a finely divided refractory material selected from the group consisting of colloidal silica and colloidal alumina, and (5) water.

5. A composition as defined in claim 4, including in percent by weight 2 to 10% chromium nitrate, 2 to 10% sodium chromate, 15 to 30% phosphoric acid, 20 to colloidal silica, and 15 to water.

6. A composition as defined in claim 5, including about 4.2% chromium nitrate, about 3.6% sodium chromate, about 25% phosphoric acid, about 28.2% colloidal silica, and about 39.0% water.

7. An insulating coating composition as defined in claim 1, wherein said trivalent chromium salt is chromium nitrate, said chromate salt is sodium chromate, and said refractory material is colloidal silica.

8. A composition as defined in claim 7, including in percent by weight about .5 to of said precipitate, about to phosphoric acid, about 10 to colloidal silica, and about 20 to Water.

9. A composition as defined in claim 8, including about 1% of said precipitate, about 27% phosphoric acid, about 30% colloidal silica, and about 42% water.

10. Magnetic sheet material having thereon a thin, tightly adherent coating composed of the cured product of the mixture defined in claim 1.

11. Coated sheet material as defined in claim 10, wherein said sheet material is constituted by strand annealed silicon steel.

12. The method of making coated magnetic sheet material which comprises providing on the surface thereof a mixture as defined in claim 1, and heating the same at a temperature of between 200 C. and 300 C.

.13. The method as defined in claim 12, wherein said sheet material is subjected to heat treatment above about 6 900 C. prior to the provision of said coating mixture thereon.

14. The method of making coated magnetic sheet material which comprises providing on the surface thereof a coating mixture as defined in claim 7, and heating the same at a temperature of between 200 C. and 300 C.

15. The method as defined in claim 14, wherein said wherein said sheet material is constituted by magnetic silicon steel which has been subjected to heat treatment above about 900 C. in single strand form prior to the provision of said coating mixture thereon.

References Cited UNITED STATES PATENTS HAROLD A NSHER,

D. J. FRITSCH, Assistant Examiner US. Cl. X.R.

Primary Examiner 

